I. Field
The present invention relates generally to communication and more specifically to techniques for managing interference for soft handoff and broadcast services in a wireless frequency hopping (FH) communication system.
II. Background
In a frequency hopping communication system, data is transmitted on different frequency subbands in different time intervals, which may be referred to as “hop periods”. These frequency subbands may be provided by orthogonal frequency division multiplexing (OFDM), other multi-carrier modulation techniques, or some other constructs. With frequency hopping, the data transmission hops from subband to subband in a pseudo-random manner. This hopping provides frequency diversity and allows the data transmission to better withstand deleterious path effects such as narrow-band interference, jamming, fading, and so on.
An orthogonal frequency division multiple access (OFDMA) system utilizes OFDM and can support multiple users simultaneously. For a frequency hopping OFDMA system, data for a given user may be sent on a “traffic” channel that is associated with a specific FH sequence. This FH sequence may be generated based on an FH function and the traffic channel number, as described below. The FH sequence indicates the specific subband to use for data transmission in each hop period. Multiple data transmissions for multiple users may be sent simultaneously on multiple traffic channels associated with different FH sequences. These FH sequences are defined to be orthogonal to one another so that only one traffic channel, and thus only one data transmission, uses each subband in each hop period. With these orthogonal FH sequences, the multiple data transmissions do not interfere with one another while enjoying the benefits of frequency diversity.
An OFDMA system may be deployed with multiple cells. A cell can refer to a base station in the system and/or the coverage area of the base station, depending on the context in which the term is used. A data transmission on a given subband in one cell acts as interference to another data transmission on the same subband in a neighboring cell. To randomize inter-cell interference, the FH sequences for each cell are typically defined to be pseudo-random with respect to the FH sequences for neighboring cells. With these pseudo-random FH sequences, interference diversity is achieved and the data transmission for a user in a given cell observes the average interference from the data transmissions for other users in other cells.
In a multi-cell OFDMA system, it is desirable to support “soft handoff”, which is a process whereby a user communicates with multiple base stations simultaneously. Soft handoff can provide spatial diversity against deleterious path effects through transmission of data to or from multiple base stations at different locations. However, soft handoff is complicated by the use of frequency hopping. This is because the FH sequences for one cell are pseudo-random, and not orthogonal, to the FH sequences for neighboring cells in order to randomize inter-cell interference. If a user is in soft handoff with multiple base stations, then the soft-handoff user may be instructed to use an FH sequence for a designated one of these multiple base stations. The data transmission for the soft-handoff user will be orthogonal to the data transmissions for other users of the designated base station but will be pseudo-random with respect to the data transmissions for the users of other base stations. The soft-handoff user would cause interference to the users of the other base stations and would also receive interference from these users. The interference degrades the performance of all affected users.
There is therefore a need in the art for techniques to manage interference for soft handoff in a frequency hopping OFDMA system.